Bongo Fury

I have had water getting into my lights for some time but it has been very minimal, I was advised to drill a couple of small holes in the lens Not exactly keen to do this - just noticed this afternoon that I have at least 1.5inches of water in the one light !!!!
Whilst I am obviously keen to get this water out, i am more worried by how it is getting in....
Any suggestions on the cause and solution would be much appreciated - if anyone living in the Caerphilly area would like to take a look for me i would be even more greatful

The tailgate lights on the post 1999 models do seem to be prone to moisture getting in them. One of mine has what looks like condensation in, but I've not had the opportunity to take the panel off to see where it's coming from.